I don't think it really matters. Schools often encourage you to keep them informed on changes and updates in your application. I think a new LOR would count as an update.
As long as the person who writes it sends the letter, it should not be a problem to add an additional letter. Although, you might want to send a school a brief letter stating that you are adding an additional letter to your file from________
can you still send an extra letter of rec, even if the maximum is 6. The thing is my school does the composite pre-med committee, and i want to send another letter of recommendation, but that would exceed the 6 limit. Can i still send an extra, and call it an "update"